Flexible and stretchable inorganic optoelectronics

Haicheng Li; Haicheng Li; Yu Cao; Yu Cao; Zhouheng Wang; Zhouheng Wang; Xue Feng; Xue Feng

doi:10.1364/OME.9.004023

1. Introduction

Optoelectronic devices are electronic devices or systems that source [1–5], detect [6–9] and control [10–12] light, usually considered a sub-field of Photonics [13]. These devices are electrical-to-optical or optical-to-electrical, which include light-emitting devices (light-emitting diode (LED), laser diodes) [14–17], photoelectric devices (photodiodes (including solar cells), phototransistors and photomultipliers) [18–24], and photoconductive devices (photoresistors, photoconductive camera tubes) [25–28]. Optoelectronic devices or systems have been enthusiastically explored for applications such as optical fiber communications [29–32], precision measuring instruments [33–35], health care [36–38], or power source [39–42]. In this century, the researches on optoelectronics focus on improving the performance as well as changing the mechanical properties. The remarkable works have been brought to the flexible and stretchable optoelectronics to research for the compatibility of biomedical systems [43–46]. The researchers have reported advancing designs or fabricating skills for flexible optoelectronics that offer not only principles on the mechanical performance of semiconductors, but also new opportunities for applications on clinics or biomedical experiments [47–49].

Traditionally, inorganic semiconductors [50–52] are premium materials for producing high-performance optoelectronics [53–58]. These inorganic semiconductor materials with active layer structures must grow on intrinsic wafers, which are rigid, bulky, hard, and stable in high temperature [59–61]. For example, the fabrication of many LEDs based on AlGaInP structure starts from epitaxial growth on gallium arsenide (GaAs) wafer [62,63]. On the other side, biomedical systems are typically deformable when suffered external stress such as squeezing, bending or stretching. The mechanical properties are totally mismatched between the conventional inorganic devices and biomedical system, which limits the optoelectronic applications in bio-related fields. As early as 1992, Gustafsson et al. fabricated a fully flexible LED using poly (ethylene terephthalate) as the substrate, soluble polyaniline as the hole-injecting electrode, a substituted poly(1,4-phenylene-vinylene) as the electroluminescent layer and calcium as the electron-injecting top contract [64]. However, the challenges for fabricating flexible and stretchable inorganic semiconductors are still unsolved. The epaxial growth of functional layers will relate to high temperature, which is unacceptable for the soft substrate made from organic materials. This limitation has been solved by the application of transfer printing technologies [65–70], which success to integrate the inorganic semiconductors to flexible organic substrates. The stretchability of semiconductors can be realized by stretchable structure, such as serpentine shape [71–73] and island-bridge structure [74–76]. The enhancement from mechanics, fabrication have improved the performance of flexible electronics. We list some papers reported on the developments of designs, fabricating techniques, devices, and applications of flexible and stretchable semiconductors, as shown in Fig. 1 [77–100]. In 2011, Kim et al. have introduced epidermal electronics incorporating electrophysiological, temperature, and strain sensors, as well as transistors, light-emitting diodes, photodetectors, radio frequency inductors, capacitors, oscillators, and rectifying diodes. All these components are configured together into “skin-like” membranes that are invisible to the skin, much like a temporary transfer tattoo [78]. In 2012, Carlson et al. summarized transfer printing techniques, ranging from the mechanics and materials aspects that govern their operation to engineering features of their use in systems with varying levels of complexity [77]. In 2016, Choi et al. summarized the soft, flexible and stretchable electronics/optoelectronics being employed in biomedicine for implantable, minimally invasive, and wearable applications [79–81]. The Applications include monitoring of electro-physiological signals from the skin, tissues, and organs. In 2017, Liu et al. reviewed flexible and stretchable electronics for wearable health monitoring [82–87]. The integrated skin-like devices were introduced in detail and exhibited some examples of biomedical applications. In 2018, Choi et al. summarized the developments of flexible quantum dot light-emitting diodes, and showed representative examples of future interactive displays with flexible form factors, including smart glasses and/or smart lens, LEDs woven into fabric and cloth for wearable displays, ultrathin displays attached to the human skin in the form of electronic tattoos, and bendable displays utilized as foldable tablets [88]. In 2019, Kim et al. introduced the works of wearable biosensors for healthcare monitoring. The representative examples of wearable biosensors are listed, such as eyeglass-based sweat sensor, mouth-based biosensor, Graphene-based wireless bacteria sensor, integrated wearable sensor arrays, wearable chemical-electrophysiological hybrid biosensor, and smart contact-lens biosensing platform [89–100].

Fig. 1. Reviews of flexible and stretchable electronics, including flexible LEDs [64]; epidermal electronics [78]; transfer printing technologies [77]; flexible and stretchable bio-electronic devices integrated with nanomaterials [79–81]; Copyright 2016, Wiley, Copyright 2008, 2014, Springer Nature; flexible and stretchable electronics [82–87]; flexible and stretchable displays [88]; Flexible Electronics; wearable biosensors for healthcare monitoring [89–100]. Figures reused with permission from the corresponding publisher.

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In this paper, we review the rapid developments of the flexible and stretchable inorganic optoelectronics [101–103]. We first introduce the mechanical and photonic analysis for designing high performance, flexible and stretchable inorganic semiconductors. Then advances in different transfer printing techniques are examined for efficiently fabricating flexible and stretchable optoelectronics. Examples of these optoelectronic devices are listed as different functionalities, materials, and structures. The applications in biomedical engineering of the flexible and stretchable optoelectronics are subsequently reviewed. Finally, we conclude with a discussion of the flexible and stretchable optoelectronics on the emerging trends, possible developments, and opportunities in this field.

2. Mechanical designs and photonic analysis for flexibility and stretchability

2.1 Mechanical designs

The devices based on inorganic materials are generally fabricated on rigid semiconductor wafers. The substrates serve as the foundation for the manufacturing process, such as impurity doping, spin coating, and annealing. Generally, these substrates are useless for improving the performance of the semiconductors. Moreover, some electronics need to reduce the substrates to improve the performance or optimize the heat dissipation. Also, the substrates limit the flexibility of the inorganic semiconductors. The relationship between bending shiftiness and thickness of inorganic semiconductors can be expressed as follows.

(1)$${EI = \frac{{Eb{h^3}}}{{12}}}$$

Where EI, E, b, h represents bending shiftiness, Young’s modulus, width and thickness of the semiconductors, respectively. This equation expresses the mechanical characters of rectangle shape semiconductors, which is the most common structure of the inorganic semiconductors. The EI is proportional to the width and the cube of thickness. This property provides a route for designing flexible inorganic semiconductors: the inorganic semiconductors such as silicon (Si) wafers are brittle and rigid, but any material can be flexible if they are thin enough.

In the past several years, some functional inorganic semiconductors have been explored in applications of flexible electronics. The most successful method for realizing flexible semiconductors is the introduction of low-dimensional inorganic semiconductors, including materials with ultra-thin films (e.g., two-dimensional layered materials), one-dimensional inorganic structures (e.g., NWs, nanotubes, nanobelts), and even 0-dimensional nanostructures (quantum dots). Figure 2(a) shows typical flexible ribbons from a Si (111) wafer. The total thickness of the ribbons is less than 1 µm [103].

Fig. 2. Mechanical designs of the flexible and stretchable inorganic semiconductors. (a) SEM image of the flexible single crystal ribbon, which thickness is less than 1 µm [105]. (b) Scheme for explaining neutral layer, and finite element simulated through-thickness [106]. (c) Schematic illustration of the process for fabricating wavy ribbons on a plastic substrate [107]. (d) SEM image of GaAs wavy ribbons on PDMS substrate [108]. (e) Some examples of wavy ribbons based on different materials. From top to bottom: Silicon, AlN, PZT, respectively [111–111]. (f) Fractal geometries as general layouts for stretchable electronics and FEM images of each structure under elastic tensile strain and the demonstration of Si NMs patterned into Peano layouts then bonded to a 40% pre-strained elastomeric substrate [112]. Figures reused with permission from the corresponding publisher.

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Complete flexible electronics include a plastic substrate, functional layers (ultra-thin semiconductors), and packaging layer, which will form a multi-layer structure eventually, as shown in Fig. 2(b) [104]. The bending strain on each layer related to the position along thickness z. Generally, a strategy called “neutral plane” is adopted for minimizing the stain loaded on the layer to protect the functional semiconductors [111]. Theoretically, the strain is zero on the neutral plane. The position of the neutral layer in a multi-layer structure can be calculated by

(2)$${\textrm{z}({\varepsilon = 0} )= \mathop \sum \limits_{i = 1}^n {E_I}{d_i}\left[ {\left( {\mathop \sum \limits_{j = 1}^i {d_j}} \right) - \frac{{{d_i}}}{2}} \right]\left /{\mathop \sum \limits_{i = 1}^n {E_i}{d_i}}\right.}$$

where z is the distance along the z-axis in Fig. 2(b), E_i and d_i are the modulus and thickness of the i^th layer. Base on the Eq. (2), we can significantly reduce the stain in semiconductors by setting them on the neutral plane. This theory has successfully demonstrated and applied in flexible electronic designs for improving bending performance.

Stretchable semiconductors can be achieved by specific designs based on flexible configurations. Individual configurations can be fabricated for yielding geometries that can be stretchable. For example, we can build buckled ribbons on a stretchable substrate for realizing stretchable inorganic semiconductors. Generally, the ribbons are coupled on the polydimethylsiloxane (PDMS) surface by Van de Waals force. This force is not very strong and can be modulated by exposing the surface of PDMS with ultraviolet light (240-260 nm) before the transfer printing process. The pre-strained technology is used to form the buckled structure, as shown in Fig. 2(c) [105]. Figure 2(d) shows the buckled ribbons of GaAs on the PDMS substrate [106]. The PDMS substrate (thickness ∼4 mm) is loaded a large uniaxial pre-strain ${\varepsilon }$, and then transfer-print the GaAs ribbons on the PDMS surface. The wavy configuration will appear when we release the stain on PDMS. The height of the buckles will increase if we use larger ${\varepsilon }$. The width and length of the initial ribbons determine the dimensions of the wavy form. For example, in terms of the simple single layer of signal crystal silicon on PDMS, the wavy configuration can be calculated by

(3)$${{\lambda _0} = \frac{{\pi h}}{{\sqrt {{\varepsilon _0}} }},\; {A_0} = h\sqrt {\frac{{{\varepsilon _{pre}}}}{{{\varepsilon _c}}} - 1} }$$

where ${{\varepsilon }_c}$, ${{\varepsilon }_{pre}}$ are the critical strain and the level of pre-strain, respectively. Here, ${{\varepsilon }_c}$ can be calculated by Poisson ratio v and Yong’s modulus E of silicon and PDMS. The wavy wavelength $({{{\lambda }_0}} )$ and amplitude and (${A_0})$ can be determined by the thickness (h) of the semiconductors [112]. The previous works have been reported a lot of inorganic ribbons based on different materials, such as Si, AlN, and ferroelectric materials, as shown in Fig. 2(e) [107–109].

Although the wavy strategy provides a universal method for achieving stretchability, the stretchability of the wavy is limited by the configuration of the waveform. Generally, the extreme of applied strain on the waveform is less than 55%. The concept in fractal geometry can be exploited in stretchable electronics for improving the stretchable performance. Figure 2(f) shows some deterministic fractal designs, including line (Koch, Peano, Hilbert), loop (Moore, Vicsek) and branch-like geometries (Greek cross) [110]. The results of the finite element method (FEM) show that the uniaxial strain can be up to 75% without crack. These structures are also very suitable for the interconnections of stretchable semiconductor units. Experiments show that Si nanowires into second-order Peano layouts can be elastically strained by 105% as well as a maximum principle strain of 1% in the silicon. These results demonstrate the mechanical enhancement of the stretchable semiconductors by using the fractal geometries.

2.2 Photonic analysis

Low dimensional materials [114–122] are easy to deform compared with their bulk counterparts, but the optical properties are also tuned by elastic strain [123]. For example, previous works have observed significant energetic red-shift of the near-band-edge emission in bent semiconductors. The flexoelectric effect can be used to fabricate devices such as optopiezotronics, but the stability of the electronic or optoelectronic performance will be an influence when bending. Fu et al. investigate the continuously varying electric bandstructure profile when ZnO nanowires (NWs) loaded an inhomogeneous elastic strain gradient, as shown in Fig. 3(a) [113]. The NWs with high quality are bent with a special designed four-point-bending setup, and the cathodoluminescence (CL) spectra are measured at low temperature (5.5 K). Although the ${D_0}{X_A}$ luminescence peak dominates the bending region of the wire, red-shift on the spectra can be observed and compared with the emission peak in the strain-free region by 33 meV. The results demonstrate near-band-edge (NBE) emission influenced by the strain in bent ZnO NWs.

Fig. 3. Photonic analysis for observing semiconductor properties with different stain. (a) Experiments and results of CL measurements in a bent ZnO NWs. Line-scanning CL spectra on the strain-free section “I” and from the pure bending region (section “II”-“IV”) with constant strain-gradient g = 1.20% µm⁻¹ at 5.5 K, and the three-dimensional graph of the NBE emission spectra [115]. Copyright 2014, Wiley. (b) Optical microscope image, 3D microstructure, and profile details of a single ribbon for three samples with different waveform [116]. The right figures show the PL spectra with continuous position intervals in a single period, normalized Raman spectra of GaAs long-wavelength optical phonon on three particular positions, and periodic band gap shift in one more period of different samples. Copyright 2016, ACS.

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The strain-photonic coupling effect in optoelectronic materials is also obvious due to the tuning of bandgap when the semiconductors are bent [124–129]. Our group (Wang et al.) reveal the continuous strain distribution in GaAs nanoribbons by applying structural buckling, as shown in Fig. 3(b) [114]. The general bending experiments can only test the optoelectronic properties (e.g. µ-Raman spectra) at a specific radius. The wavy geometry provides a route to measure continuous photoluminescence due to the different radius on the wavy. The Raman spectra show a clear bandgap shift at different location of the periodic GaAs ribbons. In the experiments, the bandgap shifts versus per unit strain are calculated by 12.8, 12.9, and 10.9 meV/% for three wavy samples with different waveform. A change rate (∼12 meV/%) is identified to modify the <001> GaAs band structure with actual stain applied on ultra-thin films. These works reveal the relationship between optoelectronic performance and strain, which are essential for the design of flexible and stretchable semiconductors in the future.

3. Fabrication strategies and transfer printing techniques

The substrate materials of flexible and stretchable electronics are generally made by organic materials, such as silicone elastomers [130–133], polyimide [134–136], and other plastic [137–142]. These materials offer high stretchability as well as excellent adhesion for loading the inorganic semiconductors. However, the specific steps for fabricating inorganic electronics relates to high temperature, which exceeds the glass-transition and/or thermal decomposition temperatures of the plastic substrate. Transfer printing techniques [143–150] are proposed for avoiding this challenge during the fabricating process of flexible and stretchable electronics.

Figure 4(a) shows a schematic illustration of the generic process flow for transfer printing solid objects, which is realized by Meitl et al [151]. and successes to transfer printing ultra-thin inorganic semiconductors from donor substrate to receiver substrate. The process begins with the preparation of a donor substrate. The functional layer can be epitaxial growth on the donor substrate by the traditional semiconductor technologies. The essential tool used here is transfer printing stamp. Generally, the stamps are made by polymeric materials. The original materials for stamps are PDMS with a smooth surface, and there are other materials used for stamps with the developments of transfer printing technology. The soft elastomeric stamps can pick up the ultrathin semiconductors by generalized adhesion forces, and most of the forces are Van der Waals interaction. The adhesion forces can be controlled by surface treatment or kinetical sensitive due to the viscoelastic behavior of the elastomer. For example, the high peel velocity (e.g., ∼10 cm/s or fast) results in strong forces to peel off the semiconductors (also called “inks”) from donor substrate, whereas low peel velocity (∼1 mm/s) is helpful to print the inks on the receiver substrate. Figure 4(b) shows a critical energy release rates (G) for the stamp/inks and semiconductor/inks interfaces [152]. The characteristic energy release rate G is related to the peel velocity. In conclusion, there are three key elements involved in a transfer printing process: substrate, ink, and stamp.

Fig. 4. Transfer printing technologies. (a) Schematic illustration of the generic process flow for a typical transfer printing process using PDMS stamp [151]. (b) Schematic diagram of critical energy release rates for the ink/substrate interface and the stamp/ink interface. The intersection of the horizontal line in the middle with the curve represents the critical peel velocity for the kinetically controlled transfer printing. The horizontal lines at the bottom and top represent very weak and strong film/substrate interfaces, respectively, corresponding to pick up only and printing only [152]. (c) Schematic illustration of the surface-treatment transfer printing process to pattern the PDMS substrate in the subtractive and addictive switchable transfer modes. The PDMS can be patterned to different shapes, as shown in right optical images [153]. (d) Octopus-inspired pads for the transfer printing process. The adhesive force is reversibly switched by the hydrogel actuation in response to the environmental temperature. The smart stamps can transfer print semiconductors with different materials, such as Si ribbons, InGaAs ribbons, or heterostructure [154]. (e) Programmable transfer printing process via automated laser writing on a micropatterned shape memory polymer stamp. The silicon and GaAs nanoribbons can be hybrid printed on PDMS, and also printed patterns of a cross [155]. Figures reused with permission from the corresponding publisher.

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From the above analysis, the modulation of G value is critical to complete a transfer printing process. Here we mark the separation at either the ink/donor stamp/ink and ink/receiver interface as $G_c^{ink/donor}$, $G_c^{stamp/ink}$ and $G_c^{ink/receiver}$. The relationship of G at different interference must satisfy the following equation.

(4)$${G_c^{\frac{{stamp}}{{ink}}} > G_c^{\frac{{ink}}{{donor}}}}$$

(5)$${G_c^{\frac{{stamp}}{{ink}}} < G_c^{\frac{{ink}}{{receiver}}}}$$

The modulation of G can be achieved by other means such as surface treatment. Wang et al. reported the switchable transfer printing (sTP) technique by the surface treatment-assisted method. With the surface treatment with different receipt of oxygen plasma (OP) and ultraviolet ozone (UVO), they realize PDMS decal inks transferred from the “soft side” to the “hard side”, and irrespective of the “soft” side coming from the PDMS stamp or the target PDMS substrate. These works demonstrate the transfer printing can be realized even the substrates and stamps made by the same materials. In their works, a PDMS stamp with square arrays of micro-posts (5 mm in diameter and 1.8 mm in height with a center-to-center separation of 10 mm), is used for the switchable transfer. The soft adjustment can be controlled by the surface oxidation. For example, OP and UVO 30 min can achieve “soft” and “hard” PDMS surface respectively. Finally, A switchable transfer is conducted on ultra-thin PDMS films (∼70 µm thick), and various patterned thin PDMS substrates are fabricated using stamps after surface oxidation, as shown in Fig. 4(c) [153].

Lee et al. provided another technique to adjust the G value to realize switchable adhesion. They demonstrate smart adhesive stamps by mimicking the muscle actuation to control pressure-induced adhesion of octopus suckers, as shown in Fig. 4(d) [154]. This mimic pad is fabricated with a hole patterned PDMS. The adhesion of the stamp pads is controlled by the thermal stimulus. When the pads stay in low temperature, the internal cavity will be small. The cavity volume will be sharply changed at a higher temperature. With the increase of the cavity volume, the pressure in the cavity is larger than the external environment value. Then the ultra-thin semiconductor will be picked up, just as resembling the actuating muscle in an octopus sucker for the reversible adhesion activity. This adhesive pad exhibits an adhesive strength of 94 kPa in response to the temperature change between 22 and 61 °C and can be used to transfer printing semiconductors such as Si and InGaAs micro-ribbon.

The easily controlled adhesion switchability is highly desired for ideal transfer printing. In previous works, our group proposed laser-induced forward transfer (LIFT) with shape memory polymer (SMP) for printing solid pixels with high resolution, as shown in Fig. 4(e) [155]. The micropatterned SMP stamps are fabricated as cone arrays. When the SMP stamp is heated above its glass transition temperature, the SMP will become soft, and the cone deformed when pressing, and the cone can be used to pick up target semiconductors. Then the cone shape is unchanged even at the room temperature. The laser can heat the cone up to glass transition temperature again, and the cone will release the semiconductors to the receiver substrate because the cone deformation decreases the contact surface with semiconductors. The transfer process can be programed by controlling the laser position and intensity by computer. It can be used for hybrid transfer printing semiconductors with different materials to a soft substrate, which provides a route for scalable integration of sophisticated electronic devices.

4. Flexible and stretchable optoelectronic devices

4.1 Light-emitting devices

Light-emitting elements are the most common and useful optoelectronic devices for displaying information. The flexible light-emitting devices include inorganic LEDs [160–162], organic LEDs (OLEDs) [163–167], plastic LEDs (PLEDs) [64,168–170] and quantum dots (QDs) [171,172]. Although the thin-films of organics obtained many promising results of flexible screens or foldable displays, the low carrier mobilities and unstable performance are still limitations for their developments. On the contrary, traditional inorganic semiconductors exhibit excellent stability and high carrier mobility. Many research works focus on bright, inorganic electroluminescent devices, such as µLEDs, arrays, and displays. Park et al. have fabricated stretchable microscale inorganic light-emitting diodes by wavy configurations [156]. They assemble small inorganic LEDs into addressable arrays on plastic substrates. The epitaxial semiconductor layers include AlInGaP quantum well structures (6-nm-thick In_0.56Ga_0.44P wells, with 6-nm-thick barriers of Al_0.25Ga_0.25In_0.5P on top and bottom), cladding films (200-nm thick layers of In_0.5Al_0.5P:Zn and In_0.5Al_0.5P:Si for the p and n sides, respectively), spreaders (800-nm-thick layers of Al_0.45Ga_0.55As:C and Al_0.45Ga_0.55As:Si for the p and n sides, respectively), and contacts (5-nm-thick layer of GaAs:C and 500-nm-thick layer of GaAs:Si for the p and n sides, respectively), for a total thickness of ∼2.523 mm, all grown on AlAs (1500-nm-thick layer of Al_0.96Ga_0.04As:Si) on a GaAs substrate. Then the functional layer is moved from a GaAs substrate to PDMS substrate. The passive matrix, stretchable inorganic LED display that uses a noncoplanar mesh configuration, on a rubber substrate is shown in Fig. 5(a). Also, the flexible LEDs array with different light wavelength can be achieved by transfer printing methods. Different from red LEDs, the blue LEDs are fabricated by gallium nitride-based (InGaN) materials, as shown in Fig, 5(b) [157]. The InGaN epitaxial stacks are grown on Si wafers with (111) orientation. The release of these LEDs from silicon substrate is utilized by a wet chemical etching process with potassium hydroxide (KOH) or tetramethylammonium hydroxide. The flexible blue light can generate white light by integrating with phosphors on PDMS. The color will be different with different thickness of phosphors, and the chromaticity follows an approximately linear path between blue and yellow, which is determined by both the blue emission of LEDs and yellow radiation of phosphor.

Fig. 5. Flexible and stretchable light-emitting devices. SEMs and optical images of the red (a) and (b) blue LED array. The red LEDs are based on AlInGaP structures (50 mm by 50 mm) created by vertical, patterned etching an epitaxial multilayer stack grown on a GaAs wafer [156]. The blue LEDs are InGaN µ-ILED arrays grown on Si (111) wafer [157]. Both of them are moved to a plastic substrate by transfer printing techniques. (c) Wearable micro light-emitting diodes integrated on a lab coat [158]. (d) RGB displays with transfer-printed assemblies of 8 µm × 15 µm inorganic light-emitting diodes [159]. (e) A stretchable high-density array of blue LEDs. There are 500*500 units connected by stretchable wire. Figures reused with permission from the corresponding publisher.

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One of the essential applications of flexible or stretchable light-emitting devices is for wearable displays. Wearable displays are a potential candidate for future information exhibitions. Lee et al. have reported wearable micro LEDs (WµLEDs) integrated with wireless power supplement [158]. The 30*30 WµLED arrays are transferred by transparent elastomeric adhesive from GaAs substrate and combined with a wireless energy transfer system fabricated on a 100% cotton fabric. The strain of WµLEDs can be reached 100%, and the array showed excellent stability under 85 °C and 85% relative humidity. This array can be stitched onto a lab coat, which demonstrates its practical applications, as shown in Fig. 5(c).

Bower et al. fabricated emissive displays by transfer printing 8 µm *15 µm inorganic light-emitting diodes [159]. The displays integrate red light ((Al)InGaP quantum wells, and were grown on GaAs substrates), green and blue µILEDs growth taking place on a Si (111) wafer. The transfer stamp is made from glass and elastomer by injection-moulding technique. The shapes of the stamp are specific design for transfer printing LEDs with a different color. The total thickness of plastic µILED display (100 × 100, 254 PPI) is ∼130 µm, as shown in Fig. 5(d).

Our group has fabricated a 500*500 units array of blue LEDs. These blue LEDs are InGaN epitaxial structure grown on a silicon substrate. Then the functional layer is transfer printed on PDMS, and the silicon substrate is etched by oxygen plasma. The LED unit is a square with a side length of 150 µm, and the spacing between the units is 10 µm. This LEDs array can emit white light with the and yellow radiation of phosphor, and the high density of LEDs makes a high resolution.

Stretchable inorganic light-emitting arrays can be worn on the location of the body that highly deformable, which provide a possibility of tattoo-like or epidermal displays [175]. These displays are designed as stretchable structures due to the use of fractal shape and island-bridge structures. Choi et al. fabricated a stretchable, active matrix (AM) inorganic LED display with rapid response time and low power consumption [173]. Most of the applied strain is about 62%, which is limited by the serpentine bridges, and the images of stretchable displays are shown in Fig. 6(a).

Fig. 6. Mechanical designs for the stretchable LED array. (a) Stretchable active-matrix inorganic light-emitting diode display. These images show uniform emission characteristics of the stretchable LED display under uniaxial strain from 0% to 40%; white lines indicate the horizontal direction [173]. (b) Stretchable LED chain with strain isolation design. The substrate can be spatio-programmed rigidity, which means the substrate is soft and stretchable expect the section of LEDs [174]. Figures reused with permission from the corresponding publisher.

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Despite the rapid developments of stretchable optoelectronics for human-machine interface, the robustness is still an unresolved challenge for commercial use of these stretchable displays. Our group works have proposed an approach that uses a new polymer substrate with programmable rigidity to realizing strain isolation, as shown in Fig. 6(b) [174]. The plastic substrate used here is spatio-programmed substrate rigidity, which Yong’s modulus can be changed by H₂O₂. A LED chain is fabricated for demonstrating the strain isolation. The substrate under LEDs is rigid status, while the other parts are stretchable. The LEDs remain operational by uniaxial strain (21% global strain, corresponding to 36% strain at the interconnects.

4.2 Photodetectors

Photodetectors, also called photosensors, are able to convert light intensity into electrical signals, which are indispensable components for the light sensor. They have been applied in a variety of emerging areas such as wearable, implantable, or printed optical devices for light detection or perception. Previous works reported various flexible and stretchable photodetectors based on inorganic materials. Here, we introduce some examples of inorganic or inorganic-organic hybrid photodetectors on a flexible and stretchable substrate.

Silicon photodetectors can be fabricated on Si-based complementary with low-cost technologies, which is attracting growing interest in recent years. Seo et al. report flexible phototransistors on single-crystalline Si nanomembrane (Si NM), as shown in Fig. 7(a) [176]. The silicon on insulation (SOI) wafer with 270 nm p-typed doped top Si layer and a 200 nm buried oxide layer. The top Si can be used for fabricating ultra-thin photoresistors, and the oxide layer acts as a sacrificial layer for transfer printing. The device exhibits stable responsivity with less than 5% variation during bending at small radii of curvatures (up to 15 mm). GaAs is another relevant material for yielding photodetectors. Lee et al. report an array of ultrathin GaAs solar microcell grown epitaxially on GaAs wafers, and then transferred on elastomeric substrates, as shown in Fig. 7(b) [177].

Fig. 7. Flexible and stretchable inorganic/inorganic-organic hybrid photodetectors. (a) Flexible phototransistors based on transferrable single-crystalline Si nanomembrane (Si NM). The device exhibits stable responsivity with less than 5% of variation under bending at small radii of curvatures (up to 15 mm) [176]. (b) Optical images of the stretchable GaAs photovoltaic modules. The photodetector array is designed as an island-bridge structure for stretchability [177]. (c) Images and switching behaviors of an intrinsically stretchable nanowire photodetector based on ZnO NM. The ZnO NM layer is embedded in the PDMS matrix, yielding a structure with all the components (electrodes and detection channels) being intrinsically stretchable [191]. (d) Silicon-organic hybrid solar cells by the vertical array of Si micro-pillars embedded into elastomeric substrates [192]. (e) Flexibility organic-inorganic perovskite solar cells. The recoverable shape polymer is utilized as a substrate of perovskite solar cell to enable complete shape recovery of the device upon sub-millimeter bending radii [193]. Figures reused with permission from the corresponding publisher.

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Nanowire materials have been explored in flexible and stretchable applications in recent years. The advantage of one-dimensional nanostructures makes these materials suitable for flexible electronics. There are several nanowires used for flexible photodetectors, and the based materials include ZnO [178,179], SnO₂ [180–182], ZnGa₂O₄ [183–185], single-wall carbon nanotubes (SWCNT) [185,186], ZnO-CdO [187–189] and ZnO-Cds [189,190]. Figure 7(c) shows the performance and deforming images of ZnO nanowire photodetector [191]. These photodetectors are mainly focused on visible-blind UV wavelength sensing, which is essential to broad range applications such as skin protection and space exploration.

Inorganic–organic hybrid composites can achieve possible band structure modulation and charge trapping effect at the interface of organic/inorganic components. These characteristics are attractive to obtain high-performance photodetectors due to the efficiently facilitate charge separation and transportation. Recently, there is a rapid development of inorganic-organic hybrid technologies for flexible or stretchable photodetectors. Figure 7(d) is an example of stretchable, bifacial Si-organic hybrid solar cells fabricated on elastomeric substrates [192]. The solar cells can be stressed up to 100% without crack due to the vertical array of Si micro-pillars designs. The using of organic materials with specific mechanical designs can achieve a delightful deformation ability. The other materials can be used for organic-inorganic hybrid photodetector include P3HT [194–196], Indium gallium zinc oxide (IGZO) [197–199], CdSe [200,201], and others.

Perovskite is a potential material to achieve inorganic-organic hybrid photodetectors, which has already demonstrated by using it to fabricate high-performance devices such as perovskite solar cells. Park et al. focus on the optimization of the flexibility of perovskite solar cell, as shown in Fig. 7(e) [193]. The 13.5 GPa elastic modulus of the perovskite is obtained, and the stress and strain distribution in the device at small bending radii (r = 1 mm and r = 0.5 mm) is observed. Plastic deformation, as well as a significant drop in the power conversion efficiency (PCE) value, is experienced at a bending radius of 0.5 mm.

4.3 Other optical devices

The flexible and stretchable photonic devices are promising solutions for precision instruments or bandwidth interconnections. Photonic devices with different functionalities have already demonstrated on rigid substrates, such as a waveguide, filters, grating device, and sensors. In recent years, researchers have reported flexible and stretchable photonic devices that use inorganic materials. Figure 8 show examples of flexible and stretchable photonic devices. Figure 8(a) illustrates a flexible photonic device that integrates waveguides, micro-mirrors, and optoelectronic components [202]. The thin foil of 150 µm thickness with embedded active optical can be realized low-loss links for function interconnecting. A strain sensor is realized by the optical resonator. Figure 8(b) shows its undeformed and stretched states [203]. The optical performance of the devices is demonstrated with 36% nominal strain. Li et al. demonstrate single-mode stretchable integrated photonic devices, as shown in Fig. 8(c) [204]. Cho et al. report a continuously tunable color filter based on a self-assembled isotopically stretchable microbead monolayer, which can be used for measuring lateral strain, as shown in Fig. 8(d) [205].

Fig. 8. flexible and stretchable photonics for interconnection and sensing. (a) Integration photonics for signal transmission. The device integrates waveguides, micro-mirrors, and optoelectronic components [202]. (b) Flexible silicon photonic devices. An optical strain sensor uses 5 µm-radius micro ring resonators on the flexible substrate [203]. (c) Single-mode stretchable integrated photonic devices using chalcogenide glass (ChG) and epoxy polymer [204]. (d) Continuously tunable color filter based on a self-assembled isotopically stretchable microbead monolayer. The spectra of the filtered light are solely controlled by external strain (up to 32% radial strain) to cover a broad visible spectrum [205]. Figures reused with permission from the corresponding publisher.

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5. Biomedical applications

In this section, we will provide some examples of the biomedical applications of the flexible and stretchable inorganic optoelectronics. The flexible sensors or devices can be useful and efficient in combination with biomedical applications due to the similar mechanical properties with skin or tissue. There are researches focus on the stretchable devices imitating biomedical organs, and also some reports of physical sensors for monitoring vital signs. Ko et al. fabricated a digital camera inspired by the arthropod eye, which is comparable in number (180) to those of the eyes of fire ants (Solenopsis fugax) and bark beetle, as shown in Fig. 9(a) [206]. The thin, silicon photodiodes (active areas d² ≈ 160 mm*160 mm) and blocking diodes consist of the matching array in an open mesh configuration with capability for matrix addressing. The patterned PDMS consists of an array of 163 16 convex microlenses (with a radius of curvature of each microlens r < 400mm). This compound eye device demonstrated the flexible and stretchable photodetectors could act as a replacement for a real eye. Zhang et al. also fabricate hemispherical electronic eye systems with origami silicon optoelectronic, as shown in Fig. 9(b) [207]. The digital image sensor reported here is fabricated by origami approach, which provides a method for fabricating three-dimensional flexible electronics.

Fig. 9. Representative flexible and stretchable inorganic optoelectronics for biomedical applications. (a) Images of components for a digital camera that takes the form of a hemispherical, apposition compound eye. The photodiode array is fabricated by silicon, and the small hemisphere lens is made by PDMS [206]. (b) Geometric origami of silicon optoelectronics for the hemispherical electronic eye. 676 polygon blocks consisting of pentagons and hexagons were mapped into a net of a subdivided half truncated icosahedron, which was then folded to form a hemisphere [207]. (c) Images and performance of the released filter membrane on a ﬂexible PDMS sheet. This filter structure consists of a series of titanium dioxide (TiO₂) and silicon dioxide (SiO₂) films with a total thickness of about 8 µm. The films are biocompatible, and no apparent signs of pathological inﬂammatory tissue responses to the filter implantation are observed after 5 weeks [208]. (d) Injectable optoelectronics with applications for wireless optogenetics. A custom flexible polyimide film–based lightweight (∼0.7 g) power scavenger or a rigid printed circuit board-based scavenger (∼2.0 g) can be temporarily mounted on freely moving animals for short-term experimentation without constraint in natural animal behavior [209]. Figures reused with permission from the corresponding publisher.

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Liu et al. report thin-film optical filters that can be used for biomedical applications due to the excellent vivo compatibility, as shown in Fig. 9(c) [208]. The filter can reflect blue and green photons (420–560 nm) and transmits yellow and red photons (560–620 nm). The materials for fabricating the filters include titanium dioxide (TiO₂) and silicon dioxide (SiO₂) films with a total thickness of about 8 µm. The package of PDMS can improve the biocompatibility, which is demonstrated by the vivo experiments. These filter membranes can combine with optoelectronic devices for further applications.

Flexible and stretchable optoelectronics are also suitable for yielding injectable or implantable devices due to similar mechanical properties with biomedical systems. Ultra-thin semiconductors with high performance are helpful for scientific research of clinics. Kim et al. introduce injectable class optoelectronics that can insert light sources, detectors, sensors, and other components into precise locations of the deep brain for optogenetic research. The gallium nitride (GaN) µ-ILED (6.45 µm thick, 50 × 50 µm²) is integrated on a flexible substrate, and the total thickness of the injected multifunctional optoelectronic systems is only ∼20 µm. This exceptionally thin geometry, low bending rigidity, and a high degree of mechanical allow for minimally invasive operation, as shown in Fig. 9(d).[209]

Flexible and stretchable optoelectronic offer a wide range of unprecedented opportunities for monitoring vital signs. The ability of synchronous deformation with skin can achieve long-term monitoring without irritation to human skin or tissue. The comprehensive capabilities of bending, stretching or twisting also provide outstanding conformability to skin. The flexible optoelectronics can monitoring pulse rate and oxygen blood saturation, which is the critical indicators for evaluating healthy [213–217]. Although there are many commercial oximeters, most of them are fabricated on traditional printed circuit board (PCB) and integrated on a smartwatch or wrist band. The stress or squeeze is loaded on the skin when wearing these devices. Also, these oximeters can work at a specific location of the body, which limits the applications of healthcare monitoring in terms of different scenarios. Flexible or stretchable optoelectronics can solve the problems mentioned above due to the deformable and imperceptible characteristics. Kim et al. report miniaturized battery-free wireless systems using inorganic semiconductors, as shown in Fig. 10(a) [210]. This reflectance oximeter using red infrared LEDs and A photodetector positioned in between these two LEDs captures the backscattered light from the blood. These measurements are possible, mostly independent of variations in the shape, thickness, and optical properties of the nail, due to sufficient signal to noise ratio and intimate contact. Operation on the fingernail is highly desirable for long-term operation, with minimized motion artifacts and risks for irritation.

Fig. 10. Flexible and stretchable oximeters. (a) Miniaturized battery-free wireless systems for wearable pulse oximetry. The wireless transmitting components are integrated with inorganic optoelectronics, and the systems can conformal contact to the target probing area (i.e., fingernail, toenail, or various skin locations) for avoiding mechanical damage [210]. (b) The performance analysis via Monte Carlo (MC) simulation. The deformation of flexible devices will cause an unstable optical path. Mechanical optimization is needed to keep the detected signal stable during deformation [211]. (c) Epidermal optoelectronics with strain isolation design for blood oxygen monitoring. The all-in-one free-floating structure is adopted to keep the stable position of LEDs and photodetectors, which stabilize the optical signals for precise blood oxygen monitoring during deformation [212]. Figures reused with permission from the corresponding publisher.

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Our group publishes the optical and mechanical optimization of the epidermal optoelectronics for monitoring blood oxygen saturation. We first simulate the optical path in human tissue for analyzing the influence of deformation to detected signals, as shown in Fig. 10(b) [211]. A 2D tissue model is built, and the distribution of exited light from human tissue is investigated by Monte Carlo (MC) simulation. The performance with different distance between LEDs and photodetector, light incident angle and intensity is quantified by simulation. Then a systematic strategy to design an epidermal inorganic optoelectronic device for blood oxygen monitoring is proposed by using specific strain-isolation design, nanodiamond thinning, and hybrid transfer printing, as shown in Fig. 10(c) [212]. The all-in-one free-floating structure can isolate the strain loaded on LEDs and photodetectors, which results in a fixed distance position between LEDs and photodetectors when stretching. The optical signals are stable when the device is deformed. These epidermal oximeters achieve not only flexibility and stretchability but also the stability of the optical path, which improves the performance of optoelectronics for biomedical application.

6. Summary and perspective

In this paper, we reviewed the latest developments of flexible and stretchable inorganic optoelectronics, including design, fabrication, devices, and the applications on biomedical applications. The combination of organic mechanical properties and inorganic functional devices provides several promising engineering options for fully integrated systems in future distributed or mixed form. The researches of stretchable devices are very complex, which involve materials growth, heterogeneous integration, geometry design and micromechanics, and adhesion and interface science. For example, the modulus of silicon is about 100,000 times that of a typical elastomer; the thermal conductivity is ∼1000 times, and the coefficient of thermal expansion is ∼100 times smaller [218]. For solving this extreme mismatch, the design of reasonable mechanical structures is required. The challenges are even more pronounced for more complex, large-scale integrated stretchable devices because the device deformation can have a massive influence on device performance. The researches on these complex systems require comprehensive consideration of materials science and effective thermal management to ensure mechanical reliability. Stretchable electronic devices must not only achieve large-scale integration but also ensure reliability in tens of thousands of stretching processes for commercial use in the future.

At present, the results of stretchable devices are changing our concept of electronics. The electronics are no longer hard and rigid but soft and stretchable as human skin. They have potential applications in many fields, and some of the most compelling applications are biomedical related devices that solve critical issues in personal health monitoring and treatment. The soft and elastic mechanical properties are more incoordinate with the actual situation of biomedical tissues. Such skin-like devices, combined with the use of biocompatible materials, significantly improve human comfort when exposed to a biomedical interface. Therefore, stretchable devices are capable of long-term monitoring tasks. At present, the main works in this area focus on integrating flexible LEDs and photodetectors and utilizing the spectral absorption characteristics of human tissues to realize vital signs monitoring. In addition to biomedical applications, stretchable electronic devices can also be used in industries such as optical communications or robotics. Even they may be used in future cellular phones, as the recently released foldable mobile phones involve the perceived form of mobile phones. The basic ideas can also be exploited in other semiconductor technologies, including electromagnetism or acoustics. These and other related engineering opportunities, together with a wide range of fascinating scientific themes, provide a powerful promotion for applications with critical social impacts.

Funding

National Basic Research Program of China (973 Program) (2015CB351900); National Natural Science Foundation of China (11320101001, 11625207).

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Flexible and stretchable inorganic optoelectronics

Abstract

1. Introduction

2. Mechanical designs and photonic analysis for flexibility and stretchability

2.1 Mechanical designs

2.2 Photonic analysis

3. Fabrication strategies and transfer printing techniques

4. Flexible and stretchable optoelectronic devices

4.1 Light-emitting devices

4.2 Photodetectors

4.3 Other optical devices

5. Biomedical applications

6. Summary and perspective

Funding

References

Cited By

Figures (10)

Equations (5)

Optical Materials Express